The threesome believe the spells in J.K. Rowling's best-selling fantasy series are real, and dangerous. They see Britain as a hotbed of occult activity whose origins go back to pagan times, according to the Daily Mail.

The young women are motivated by their Christian faith.

"For only through the power of God, and in the name of Jesus could a 5’ 4” seventeen-year-old, young woman successfully cast out demons!" said Tess Scherkenback on her Facebook page, TeenageExorcists (http://on.fb.me/1fShR0S).

Their teacher is Brynne's father, the Rev. Bob Larson, who says he has performed more than 10,000 exorcisms in the last 30 years.

But Larson claims that 50 percent of the population is probably affected by demons in some way and his girls are the front line of defense. The girls summon the demon within the subject, and then the demon apparently takes over the person's body. Brynne said she can tell when someone is demonically possessed when she looks into his eyes.

"When you look at that person, you could just see the evil looking back," she said.

BBC3 is airing a documentary on the trio this week: "Now they are taking the fight to a city they think of as one of the most spiritually corrupt in the world - London. But what will Brits make of these evangelical American exorcists?"

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